simulation@ufl.edu (SIMULATION MODELING & ANALYSIS) (05/08/88)
Volume: 2, Issue: 3, Sat May 7 22:28:28 EDT 1988 +----------------+ | TODAY'S TOPICS | +----------------+ (1) The Viability of Distributed Simulation ---------------------------------------------------------------------------- Date: Fri, 6 May 88 18:00:48 EDT From: Paul Fishwick <fishwick@fish.cis.ufl.edu> To: simulation@ufl.edu In Vol. 2, Num. 1, Jerry Freedman writes: > Distributed Simulation is a waste of time.period. > I have been working with communication network simulations > for about 6 years. Most of what follow has NO theoretical basis > at all and is entirely pragmatic. First, I think it's necessary to define what is meant by "distributed simulation." I'm in favor of taking the most general view that suggests that distributed simulation refers to simulation on more than one node. Nodes may be highly localized as in an array processor or neural network, or they may be remote as in a network of satellites. If we take the "local view", then distributed simulation not only makes sense, but it is downright necessary since all architectures in another several years will contain many processors. I'm not talking of five or ten, but thousands. Most current ones already do have several; however, the user cannot always access all parallel nodes (i.e. internal pipelining). So, which type of "distributed simulation" do you mean? : 1) a simulation run on a parallel computer? 2) a simulation run on a local area network (say a net of *reliable* SUN's)? 3) a simulation run on a large area network? 4) or even, a simulation of a distributed system run on a single node? (#4 being the most common type of distributed simulation, I suppose) > It is almost impossible to guarantee that > the messages will always be transmitted, arrive and be processed > in the same order every time given the same scenario and input > parameters. (Please hold the flames on sophiticated tracing, > distributed debugging etc. I'm talking pragmatics here - not > the lates ACM published academic vapor). Determinism is clearly the question here. Guaranteeing determinism can be difficult if the processing nodes are far apart and subject to the whims of the local telephone company. I agree that simulation spread among many distant nodes (type #3 above) is a problem and its usefulness is questionable. > Now, I claim the the largest number of threads of control ( > cpus, tasks, whatever you may call them ) you could possibly > have any hope to follow and understand (or purchase ) is > 10. The best you could do in a perfect world is a speed up > factor of ten. But the world is imperfect- you probably couldn't > do better that 3, 5 at best. Hardly worth it in light > of the price paid (see above). What are your reasons for magic numbers like 10? Depending on the overhead associated with message passing, considerable speedups are possible with parallel architectures. > waiting a couple of years(you would have spent this time > building, testing and verfiying your distributed monster) and buying a > faster chip with a fancier compiler. How about this for a scenario? : Perhaps the "faster chip" is, in fact, a multi processor chip (with, say, 1000 fine grain processors located in it). Now, I wonder how a company might come out with such a chip without performing (yes, its coming!!) many tests on it first. What kinds of tests would they perform? How about analysis based on results from a *distributed simulation*?! > DISCLAIMER: Any snide comments in the above are semi-intentional. > I don't want to tick anyone off - I'd just like to stimulate > some discussion. I'm glad you made this posting and I hope that others will offer their thoughts as freely as you have. -paul fishwick +------------------------------------------------------------------------+ | Paul A. Fishwick.......... INTERNET: fishwick@uflorida.cis.ufl.edu | | Dept. of Computer Science. UUCP: {gatech|ihnp4}!codas!uflorida!fishwick| | Univ. of Florida.......... PHONE: (904)-335-8036 | | Bldg. CSE, Room 301....... FACS is available | | Gainesville, FL 32611..... | +------------------------------------------------------------------------+ +--------------------------+ | END OF SIMULATION DIGEST | +--------------------------+